Atherosclerotic lesions arise following severe inflammatory response to a variety of insults. The abnormal pathology associated with this disease likely arises from an abnormal growth factor and cytokine expression, excessive cellular processing of lipoproteins, and altered proteinase activity, all of which result in increased smooth muscle cell migration and proliferation. The low density lipoprotein receptor related protein (LRP) is a large endocytic multi-ligand receptor that mediates the hepatic uptake of circulating chylomicron remnants, serpin- enzyme complexes, and proteinases of the fibrinolytic pathway. LRP is expressed not only in hepatocytes, but also in macrophages, smooth muscle cells, fibroblasts, and neurons, suggesting that it is involved in the binding and removal of interstitial ligands (eg. proteinases and lipoproteins) produced by these cells. A 39 kDa receptor associated protein (RAP) binds reversibly to LRP and antagonizes its ligand binding. RAP is found primarily in the endoplasmic reticulum, where it is thought to function as a molecular chaperone for LRP, gp330/megalin, and the VLDL receptor. The central hypothesis of this application is that regulation of proteinase activity and lipoprotein levels by LRP represents an important physiological pathway, and that the activity of LRP contributes to certain processes such as cell migration and to the pathology of certain diseases such as atherosclerosis. The specific hypotheses to be tested are: 1) That RAP modulates ligand binding to LRP by an allosteric mechanism 2) that RAP plays an important role in the expression of LRP activity by minimizing ligand-induced LRP degradation 3) that LRP may regulate cell surface proteinase activity, and 4) that LRP activity is regulated by its phosphorylation. These hypothesis will be tested in the following aims: 1. Identify the regions on LRP responsible for RAP and ligand binding 2. Define the mechanism of ligand induced degradation of LRP and the role of RAP in minimizing this process 3. Define the mechanisms by which LRP regulates cellular proteinase activity, and determine the contribution of this pathway to such processes as cell migration. 4. Determine if LRP is phosphorylated and if so, define the significance of this phosphorylation.